Question

A combination is made of two lenses of focal length f and f'
in contact, the dispersive powers of the materials of the
lenses are ⍵ and ⍵'. The combination is achromatic, when
(a) ⍵ = ⍵₀, ⍵' = 2⍵₀ f ¢ = 2f
(b) ⍵ = ⍵₀, ⍵' = 2⍵₀ f ¢ = f/2
(c) ⍵ = ⍵₀, ⍵' = 2⍵₀ f ¢ = –f/2
(d) ⍵ = ⍵₀, ⍵' = 2⍵₀ f ¢ = –2f

Answer 1

Explanation:

a) is the right answer.